Known converter circuits comprise a converter unit with a multiplicity of controllable power semiconductor switches, which are connected in a known way to the circuit of at least two switching voltage levels. A converter circuit of this type is shown in FIG. 1. At the AC voltage end, the converter unit 1 is connected to an electrical AC voltage network 2. In addition, a capacitive energy store 3 is connected to the converter unit 1, which is currently made up of one or more capacitors. In order to operate the converter circuit, a control device 4 is typically provided, which controls the controllable power semiconductor switches by means of a control signal S according to the method during operation of the converter circuit, wherein the control signal S is generated by means of a look-up table, in which control signals are permanently assigned to corresponding reference values, or by means of a modulator, which is based on pulse width modulation, particularly when the converter circuit is being operated in normal mode.
The problem with a converter circuit of this type is that where there are asymmetrical phase voltages UNa, UNb, UNc in the AC voltage network 2, i.e. different phase angles of the phase voltages UNa, UNb, UNc relative to one another and/or different amplitudes of the phase voltages UNa, UNb, UNc to one another and/or different frequencies of the phase voltages UNa, UNb, UNc to one another, the converter unit's power semiconductor switch may be damaged or even destroyed, since with this sort of asymmetry, for example, unwanted currents flow across the power semiconductor. This means that the availability of the converter circuit drops massively and maintenance of the converter circuit rises significantly.